Pump swashplate control



Nov. 1, 1966 H. c. MOON, JR 3,282,225

PUMP SWASHPLATE CONTROL Filed June 4, 1965 5 Sheets-Sheet l Nov. 1, 1966 H. c. MOON, JR

PUMP SWASHPLATE CONTROL 5 Sheets-Sheet 2 Filed June 4, 1965 WNW www

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Nov. 1, 1966 H. c. MOON, JR

PUMP SWASHPLATE CONTROL 5 Sheets-Sheet 3 Filed June 4, 1965 United States Patent O a,2s2,225 PUMP SWASHPLATE CNTROL Harry C. Moon, lir., Rockford, lill., assigner to Sunstrand Corporation, a corporation of Illinois Filed `haue 4, 1965, Ser. No. 461,356 12 Claims. (Cl. 1031-162) This invent-ion relates generally to hydraulic energy translating devices and more particularly to a pressurelimiting control device for 'a multiple piston hydraulic pump, and this application is a continuation-impart of my copending application Serial No. 379,968 filed July 2, 1964, now abandoned.

In axial piston hydraulic pumps, a cylinder block with a plurality of reciprocating pistons rotates against a stationary valve plate having two arcuate ports therein, one of which is the high pressure discharge port, depending upon the direction of rotation of the cylinder block, and the other the low pressure intake port. A relatively stationary swashplate or cam member conventionally engages the ends of the pistons and reciprocates the pistons as the cylinder block rotates thereby compressing and delivering iluid through the high pressure port and drawing fluid into the cylinders through the low pressure port. It is desirable in this type ofY hydraulic pump to prevent the pump from delivering uid in excess of some predetermined maximum pressure determined in accordance with the systems limitations.

lt is, therefore, a primary object of the present invention to provide a pressure-limiting control for a variable displacement hydraulic unit.

Two major forces act to rotate the swashplate about its pivotal axis during pumping. These forces are the piston inertia moment and the hydraulic pressure moment, the former acting in a stroke-increasing direction on the swashplate and the latter acting in a stroke-decreasing direction on the swashplate. Another object of the present invention is to spring-balance the piston inertia moment and the fluid pressure moment acting on the swashplate in all adjusted positions thereof so that a system pressure in excess of the predetermined maximum will cause the swashplate to move toward the zero displacement position thereby reducing the system pressure below the excessive value.

-A further object of the present invention is to provide an adjustable stop abutment and a tension spring both engaging the swashplate which serve to limit the maximum pressure delivered by a reciprocating piston pump.

A still further object of the present invention is to provide a torsion type spring mounted on a reversible swashplate in a multiple piston pump engaging a positioning member for the swashplate to limit the maximum permissible pressure delivered by the hydraulic pump.

A more specific object of the present invention is to provide a hydraulic pump having a valve member with inlet and outlet ports, a cylinder block rotatable relative to said valve member and having cylinders therein successively communicating with said ports, pistons slidable in the cylinders for receiving and expelling fluid through the valve ports, a pivotally mounted cam member aidjacent the cylinder block and'connected to the pistons to reciprocate the pistons, the cam member being movable to a Zero stroke position and a maximum stroke position, and a pressure-limiting device for preventing excessive pumping pressures including an adjustable threaded stop engaging a portion of the cam member and preventing movement of the cam member toward the maximum stroke position, and a coil spring connected to said cam member and urging said cam member against the stop, the pistons exerting a iiuid pressure moment against the cam member urging .the cam towards the ice Zero stroke position and exerting an inertia moment on the cam member urging the cam toward the maximum stroke position, the coil spring having sufficient torce to exert a moment on the cam member equal to the uid pressure moment at the maximum fluid pressure at the zero stroke position of the cam member, the cam spring y having suiiicient force to exert a moment on the cam member equal to the algebraic sum of the inertia moment and the iiuid pressure moment throughout the range of movement of the cam member whereby the cam will move away from the stop when the maximum desired iiuid pressure is exceeded regardless of the initial position of the swashplate.

Another specific object of the present invention is to provide a reversible hydraulic transmission having a multiple piston pump with inlet and outlet ports, a motor having inlet and outlet ports, conduits connecting the ports each adapted to carry high pressure fluid, a pivotally mounted cam member in the pump adapted to reciprocate the pistons, the cam member having la zero stroke position, and maximum stroke positions on either side of the zero stroke position whereby either of the conduits may deliver high pressure uid, a pressure-limiting device for limiting the maximum uid pressure in the conduits including a torsion coil spring mounted on the pivotal axis of said cam member and having spring arms extending therefrom and crossing one another, a spacer between said arms for maintaining said arms apart, an adjustable threaded member having an annular abutment engaging each of said spring arms whereby the cam member may be adjusted and excessive conduit pressure will cause the cam member to move toward said zero stroke position.

A further object of the present invention is to provide a reversible hydraulic energy translating device of the type having -a rotating cylinder block with axially disposed pistons and a cam member therein adapted to reciprocate the pistons, the cam member having a Zero stroke position, and maximum stroke positions on either side of the zerostroke position whereby either of the ports may carry the high pressure liuid together with a pressurelimiting device. for limiting the maximum uid pressure in the ports including a torsion coil spring mounted to move with the cam member and arranged about the pivotal axis thereof, the torsion spring having its ends' normally engaging a spacing projection carried by the earn member, and a manual control handle pivotally mounted on the axis of the cam member but being free to move with respect thereto, the manual handle lying adjacent the projection and also being ilanked by the ends of the torsion spring, whereby the manual handle normally positions the cam member through the spring ends but in the event of an excessive fluid moment on the cam member by the pistons, the cam member will move in a .stroke-decreasing direction against the opposing spring force while the manual handle will remain in its originally set position.

Other objects and advantages will become readily apparent from the following detailed description taken in connection with the accompanying drawings, in which:

FIG. 1 is a partially schematic elevation view of a hydraulic unit including the tension spring embodiment of the invention;

FIG. 2 is a cross sectional elevation of a swashplate subassembly employed in the present invention;

FIG. 3 is a plan View of the valve plate in the pump of FIG. 1 but in a plane rotated 180 degrees from the plane of FIG. 1, with a pressure curve pattern developed around the periphery thereof;

FIG. 4 isa graph showing the various moments acting on the swashplate in the present invention;

FIG. 5 is a hydraulic transmission incorporating another embodiment of the present invention which permits pressure-limiting control on the swashplate even though the swashplate is reversed;

FIG. 6 is a partly diagrammatic, partly sectional view of a transmission incorporating still another embodiment of the present invention;

FIG. 7 is a cross section taken `generally along linev 7 7 of FIG. 6 showing the displacement cam mounting and control handle; and

FIG. 8 is a fragmentary view taken generally along line 8 8 of FIG. 7 showing the control handle mountmg.

While illustrative embodiments of the present invention are shown in the drawings and will be described in detail herein, the invention is susceptible of embodiments in many different forms and it should be understood that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the invention to the embodiments illustrated. The scope of the invention will be pointed out in the appended claims.

Viewing FIG. 1 wherein a hydraulic pump is generally indicated by the numeral 10, it should be understood that the pump is exemplied in a purely schematic form and is not intended to be a correct detail of all the parts therein. A supporting member 11 has two passages 12 therein for delivering fluid to the load and receiving fluid from the load or a reservoir. Fixed to the supporting member 11 is a port plate 13, -shown more clearly in FIG. 3, having an arcuate inlet port 15 and an arcuate outlet port 16 with crossover lands 17 and 18 between the ports 15 and 16. A cylinder block 20 is rotatably mounted in a housing (not shown) and has a face rotatably engaging the face of the valve plate 13. A plurality of cylinders 21 in the cylinder block 20 successively communicate with the ports 15 and 16 as the cylinder block 20 rotates.

Slidably mounted in the cylinders 21 are pistons 22, shown more clearly in FIG. 2. Each piston 22 has a spherical ball 23 on the end thereof each mounted within a spherical socket in the slippers 24. The slippers 24 are slidably mounted on an annular plate 26 rotatable on the swashplate or cam member 27. The swashplate 27 is pivotally mounted in the housing (not shown) about axis 30 which intersects the axis of the cylinder block at the place where the cylinder block axis intersects the plane containing the centers of the spherical piston ends.

A pressure-limiting device, generally indicated by the numeral 35 in FIG. 1, permits the swashplate 27, shown near its maximum displacement position in FIG. 1, to rotate toward its zero displacement position, i.e., vertical, when the pressure in the high pressure passage or port 16 exceeds some predetermined maximum value. The pressure-limiting means 35 includes a threaded support 36 fixed to the housing (not shown) which receives a threaded stop 37, adjustable by thumb screw 38 which engages a projection 40 on the swashplate 27 and permits the swashplate to be adjusted on one side of the zero stroke position. The swashplate in the embodiment shown in FIG. 1 is not reversible and therefore the high and low pressure ports 15 and 16 may not be reversed. For any adjusted position of the swashplate 27 as dictated by the adjustable stop 37, the stop 37 prevents the swashplate from pivoting in a stroke-increasing direction indicated by the numeral 42. A coiled tension spring 43 hooked to the support 36 at oneend and the projection 40 at the other end urges the projection 40 against the stop 37 and tends t-o urge the swashplate 27 in a strokeincreasing direction.

An important feature of the present invention is that the spring 43 will at all adjusted positions of the swashplate determined by the stop 37, just balance the other moments acting on the swashplate about the pivot 30 when the pump is pumping at maximum pressure so that any increase in pressure above the maximum will rotate the swashplate in a stroke-decreasing direction, indicated by the numeral 44, thereby reducing the displacement of the pump and reducing the pressure of the iluid delivered. For a complete understanding of how the spring 43 is designed to accomplish -this function, it is necessary to understand the moments acting on the swashplate about the pivot 30 during rotation -of the cylinder block. There are two major forces acting on the swashplate 27 during rotation of the cylinder block 29, and these will be described with respect to FIGS. 2 and 3. Referring to FIG. 2, during the pumping stroke of the pistons, as the pistons approach the top dead center position, slippers 24, which are retained against the swashplate 27, rapidly decelerate the pistons 22. This causes a force acting on the swashplate 27 along a line extending through the center of the piston in the top dead center position due to the inertia force of the piston decelerating. This inertia `force causes a moment acting on the swashplate 27 about axis 311 which tends to increase the stroke of the swashplate 27 and urge it toward its maximum stroke position.

Referring to FIG. 3, a pressure graph 50 is developed about the valve plate 18 and indicates the pressure within the cylinders 20 as the cylinder block rotates in the direction of arrow 52 on the swashplate. It has been found from experimentation that as the cylinders pass over the top dead center position of the crossover 17 and begin moving into the cylinder toward the port plate that the pressure in the cylinder does not immediately reach maximum value because of the slight compressibility of the hydraulic uid and also because of the crossover 17 itself. This produces a gradual rise in the cylinder pressure, as indicated at 55, on the pressure curve 50. A somewhat similar curve results on the crossover between the high pressure port 16 and the low pressure port 15. As indicated at 56, the pressure in the cylinders as the cylinder passes bottom dead center does not immediately decrease to inlet pressure but does so relatively gradually, though the pressure drop occurs more quickly than the pressure increase. Because the net fluid pressure forces in the cylinders are greater below the axis 30 than above the axis 30, as indicated by the graph of FIG. 3, the fluid pressure in the cylinders 20 acting through the pistons 22 exerts a net fluid pressure moment on the swashplate 27 which acts in a stroke-reducing direction indicated by the numeral 44 in FIGS. 1 and 2. A more detailed discussion of these forces is contained in the Hann et al. application Serial No. 113,697, filed May 31, 1961.

These two moments described above are plotted on the graph of FIG. 4 for various swashplate angles. The uid pressure moment is stroke-reducing, as noted above, and increases slightly as the swashplate angle increases. This increase is due to increased reaction force on the swashplate and increase in moment arm as the swashplate moves out of a plane perpendicular to the axis of the cylinder block. The inertia moment, discussed above, is zero at zero swashplate angle because the pistons are not reciprocating, and is a maximum at the maximum swashplate angle as the pistons travel at the greatest speed at this time. It should be noted that the graph in FIG. 4 is drawn for a constant pump speed.

. The tension in spring 43 is designed so that it exerts a moment on the swashplate at zero displacement just equal to the fluid pressure moment, as shown in FIG. 4, i.e., both about 53 inch-pounds. The spring rate of the spring 43 is chosen so that the moment exerted by the spring on the swashplate at maximum swashplate angle just equals the algebraic sum of the inertia moment and the uid pressure moment, i.e., about 40 inch-pounds. As the moments are approximately linear with swashplate angle, the spring moment balances the net amount of the inertial and fluid pressure moments in all swashplate angles for a particular pump speed. Regardless of swashplate angle, if the uid pressure exceeds the maximum fluid pressure for which the spring 43 Was designed, the stroke reducing fluid pressure moment will rotate the swashplate in direction 44 of FIG. 2, i.e., stroke reducing and reduce the displacement ofl the hydraulic pump thereby decreasing the fluid pressure in the high pressure passage 16, as a lesser volume of d uid is delivered by the pump.

A second embodiment of the present invention is shown in FIG. 5, incorporated in a closed circuit hydraulic transmission. The transmission includes a main pump with an input shaft 6() connected to a rotating cylinder block 61 having pistons 62 slidable therein and connected to the swashplate 63 in the same manner as shown in FIG. 2. lt should be understood that the transmission in FIG. 5 is merely schematic and not intended to show detailed construction. Cylinder block 61 is rotatably mounted on a port or Vvalve plate 65 having inlet and outlet ports in the same fashion as port plate 13 in FIG. 3. Grooves 66 in the cylinder block 61 and the port plate 65 deine a centrifugal charge pump which supplies make-up fluid to Athe closed system. This make-up pump is described in more detail in the Reinke application, Serial No. 338,177, iiled January 16, 1964, now Patent No. 3,177,666.

T he main pump delivers high pressure fluid to a motor 69 through an outlet port in valve plate 65 and conduit 7i). The motor 69 is shown as an axial piston motor. However, it is to be understood that other types of motors, such as gear motors, could be equally well employed. Low pressure iiuid is returned from the motor 69 to the pump 61 through a conduit 72 connected to the pump inlet passage port.

A make-up and relief and excess uid discharge shuttle valve 75 ports heated Huid from the low pressure passage 72 through tube 76 and supplies make-up fluid from the charge pump through groove 77 to the inlet of the main pump. The shuttle valve '75 is reversible depending upon which of the conduits 70 or 72 carries the high pressure uid from the main pump.

lt should be understood that the main pump is reverl sible and that either passage 7i or 72 may be the high pressure one. A shaft 80, threaded into a support member 31 in the housing (not shown) has an annular abutment 82 xed thereon at one end. The shaft Si) is vertically adjustable by the manual rotation of a handle 84. The swashplate 63 is pivotally mounted in the housing (not shown) on a stationary trunnion shaft 86 which rotatably supports a boss 87 formed on the svvashplate 63. A coiled torsion spring 83 is positioned on the boss 87 and has spring arms 39 and 90 which engage and flank both sides of the annular abutment 82. A cylindrical pin 94 lixed on an arm 9S rigid with the svvashplate maintains the spring arms 39 and 9@ in a crossed arrangement and causes pivotal movement of the swashplate with adjustment of the shaft 80. When the shaft Si) moves vertically the torsion spring 88 resiliently constrains the swashplate 63 to move with the shaft Si). At the same time, the swashplate may yield toward neutral in a manner similar to the embodiment of FIG. l. The swashplate 63 is reversible and the spring 88 is designed to limit the maximum system pressure in either conduit 70 or 72 depending upon whether the swashplate is in a positive or negative position.

Slightly different design considerations are incorporated into the pressure limiting control device in the FIG. 5 embodiment. Preferably, pistons 62 are constructed of a lighter material than is conventionally employed in order to reduce the inertia moment of maximum displacement so that the maximum inertia moment approximately equals the change in maximum fluid pressure moment from zero displacement to maximum displacement, the latter being about five inch-pounds, as shown in FIG. 4 at 100. Secondly, the spring rate in spring 88 is substantially constant for all swashplate angles, i.e., about 53 inch-pounds in the example shown. In this Way, the stroke increasing inertia moment 161 cancels the increase in the fluid pressure from zero to maximum displacement and the spring moment balances the resulting iiuid pressure moment. Therefore, if the pressure in one of the pressure passages 7@ or 72 increases above the maximum fluid pressure for which the spring 8S was designed, the swashplate 63 will rotate in a stroke reducing direction 44 and thereby reduce the iiow delivered by the pump and reduce the pressure in the outlet passage to the desired maxnnum.

A third embodiment of the present invention is shown in FIGS. 6 to 8 incorporated into a hydrostatic transmission. This embodiment is conceptually similar to the embodiment of FIG. 5 and employs a similar torsion spring carried by the displacement cam member which permits the cam member to move in a stroke decreasing direction under an excessive iiuid pressure moment regardless of Whether the cam member is on the positive or negative side of its neutral position.

Referringrnovv to FIG. 6 in more detail, it illustrates a hydrostatictransmission including an axial piston, swashplate pump 31@ and a gear motor 311 connected in a closed reversible hydraulic circuit so that the pump is adapted to deliver fluid under pressure to drive the motor in opposite directions, and the motor returns fluid to the pump.

As illustrated, the pump 310 preferably comprises a housing 312 having a port plate 313 at one end rotatably supporting a drive shaft 314 through the medium of suitable bearing means 315. rll`he shaft 314 is supported at the opposite end of the housing by means of bearing 317 in the housing, and a shaft seal as at 319 prevents leakage along the shaft.

A rotatable cylinder block 322 is positioned in the housing 312 and has one end abutting the port plate 313. The cylinder block surrounds the central portion of the drive shaft and is suitably keyed or splined to the drive shaft as at 324 so that the cylinder block rotates with the shaft. A coiled compression spring 326 surrounds t-he shaft 314 and is positioned in a central recess inthe cylinder block to bias the cylinder block toward the port plate, and for such purpose, one end of the spring bears against a spring seat 328 engaging a snap ring 329 retained in the cylinder block, and the other end of the spring engages a collar 330 abutting a shoulder 331 on the shaft 314.

The cylinder block 322 is formed With an annular series of parallel axially disposed cylinders 334 eachhaving an inlet and outlet port 335 adjacent the port plate 313 adapted to communicate successively with inlet and outlet ports 337 and 333 in the port plate 313 on rotation of the cylinder block. The porting .335, 337, 333 may be considered conventional for the purposes of this invention, and it will be understood that the ports 337 and 338 comprise a pair of arcuately shaped apertures disposed generally symmetrically on opposite sides of the plane of the drawing which may be described as the piston dead center plane, that is, the plane in which the pistons reach opposite ends of their strokes. As will appear, either of the ports 337 and 338 may function as an inlet port and the other as an outlet port.

The cylinders 334 each receive a reciprocable piston 340 having a spherical outer end portion 341, and the spherical ends 341 are positioned respectively in bearing shoes or slippers 342 engaging an annular face plate 344 rotatable on a svvashplate 346. The svvashplate 346 is suitably mounted for pivotal movement about a pivotal axis as at 348 normal to the axis of the shaft 314 and normal to the plane of the drawing so that the swashplate may be pivoted in opposite directions from a neutral, center, nostroke position normal to the pistons as by means of a handle 35). The slippers 342 are preferably held against the face plate 344 by a retainer 352 having a central portion engaging the outer spherical surface 354 of a projection on the cylinder block.

It will be understood that the left end of the shaft 314 is adapted for connection with a suitable driving means such as an internal combustion engine adapted to rotate the shaft in a predetermined direction. With the swashplate 346 pivoted to the left as shown in the drawing, the pump is adapted to deliver fluid under pressure through the port 338 and to take iiuid in through the port 337 as the cylinder block rotates. The swashplate may be pivoted over center toward the right to cause delivery of fluid under pressure through the port 337 to drive motor 311 in a reverse direction.

The motor 311 comprises a pair of pinion-like gears 360 with teeth thereon in mesh so that upon delivery of liuid under pressure through ports 362 the gears are caused to rotate, the upper one clockwise and the lower one counterclockwise, with iiuid discharged through ports 364. The gears rotate respectively in appropriate cavities as at 366, and are suitably keyed respectively to shaft 36S from which output power may be taken as for driving a propelling mechanism for a vehicle or the like. It will be understood that if iiuid is delivered under pressure through the ports 364, the gears are driven in opposite directions with uid discharged through ports 362.

The pump and motor are connected in a closed hydraulic circuit including a conduit 370 connecting pump port 33S with motor ports 362 and a conduit 372 connecting pump port 337 with motor ports 364 so that the motor may be driven in either direction.

Charge pressure is maintained on the low pressure side of the closed hydraulic circuit and leakage fluid is replenished by a make-up pump 374 of a conventional gear type including an inner pinion-like gear 376 on a drive shaft 377 and `meshing with an internal ring-like gear member 378. The shaft 377 is adapted to be driven by the same prime mover as that driving the main pump 310 so as to draw uid through an intake passage 379 and deliver uid to a passage 380. The passage 379 communicates With a reservoir 382 with a strainer 384 therein.

Delivery passage 330 communicates with branch passages 386 and 338 leading respectively to the main conduits 370 and 372 through spring-biased check valves 390 and 392 respectively. The arrangement is such that liuid in the high pressure conduit of the conduits 370 and 372 (370 as illustrated) maintains the associated check valve (390 as illustrated) closed, and the pressure of liuid from the make-up pump opens the other check valve to deliver make-up uid to the low pressure side of the circuit.

Make-up fluid in excess of that required for make-up and charge purposes flows through a passage 394 to a spring-biased make-up relief valve 396 from which liuid iiows through a passage 398 to the interior of the pump housing 312. The fluid liows from the pump housing 312 through a passage 300 to a filter apparatus 332, and from there through a passage 304 to the reservoir 382. Leakage fluid from the motor 311 ows through a passage 394a to the passage 394.

If desired, iiuid may be taken from the make-up circuit port 366 for use in a suitable piston and cylinder device (not shown) for operating auxiliary equipment on the vehicle, and returned through an optionally plugged port 308.

In order to permit free wheeling of the vehicle so that it may be pushed about manually when desired, a needle valve or the like may be provided as at 460 for connecting a passage 461 and a passage 462. The passage 461 communicates with conduit 370 and the passage 462 communicates with conduit 372 and parallel with the passages through the motor 311. With this arrangement, when the vehicle is pushed, driving the motor 311 as a pump, the fluid pumped may `be circulated through the manually operable needle valve without going through the pump, so that the vehicle may be pushed easily.

Referring now to FIGS. 7 and 8 wherein a pressurelimitingr mechanism 420 is provided for the transmission shown in FIG. 6, connecting the manual pump control handle 350 to the pump swashplate or cam member 346, the cam member 346 has oppositely extending trunnions 421 and 422 fixed thereto by pins 423 and 424 impaling both members. The trunnions 421 and 422 are mounted for rotation in bearings 425 and 426 seated in suitable horizontally extending bores 428 and 429 in the pump housing 312. End caps 430 and 431 are provided for closing and sealing bores 428 and 429 respectively. The end caps are fixed to the housing by suitable threaded fasteners 433.

The trunnion 422 extends through a suitable bore in the end cap 431 and has a threaded stud 434 extending from the end thereof. 'A cylindrical boss 436 surrounds the end of the trunnion 422 and is lixed thereto by nut 437 threadedly engaging the stud 434. The boss 436 is rotationally fixed to the trunnion 422 by a key 438 interengaging suitable slots in the trunnion 422 and the boss 436.

As may be seen more clearly in FIG. 8, the cylindrical boss 436 has a projecting spacer member 439 fixed thereto and extending vertically therefrom when the cam member is in the neutral position. Each side of the spacer member 439 has a groove therein as shown at 440 and 441. The handle 350 includes a lever 442 which has a cylindrical lower end portion 443 freely rotatable mounted on the cylindrical boss 436. The lever 442 has arcuate grooves 444, one in each side thereof which are similar shape to the grooves 440 and 441 in the spacer projection 439. The grooves in each side of the projecting spacer 439 and the lever 442 are aligned with one another when the spacer and lever are aligned.

A coiled torsion spring 446 loosely surrounds the cylindrical boss 436 adjacent the right surface of the handle lever 442 as shown in FIG. 7. The torsion spring 446 has upwardly extending arms 448 and 449 which cross in a manner shown in FIG. 8. The ends 450 and 451 of the spring arms 44S and 449 respectively, extend horizontally and are parallel to each other engaging the grooves in the sides of lever 442 and projecting spacer 439. The ends 450 and 451 are urged toward each other by the force of the spring 446 and normally maintain the projecting spacer 439 in alignment with the handle lever 442 so that movement of the handle 350 normally produces a corresponding angular movement of cam member 346.

An excessive uid pressure moment on the cam member 346 through the pistons 340 will cause the cam member, the trunnion 422 and the projecting spacer 439 to rotate in a stroke reducing direction overcoming the force of the spring 446 permitting the spacer 439 to move away from the lever 442. This may occur when the operator rotates the control handle 350 rapidly in a stroke increasing direction. It should be noted that the cam member 346 is reversible and that the torsion spring 446 permits the cam member to yield toward' the neutral position from an initially preset position on either side of neutral. In this manner, the resilient pressure-limiting control 420 is reversible in the sense that it permits the cam member to move in a stroke-decreasing direction when excessive pressure is produced in the hydraulic circuit regardless of which port in the valve plate is the high pressure port.

The torsion spring 446 will, at all adjusted positions of the cam member, just balance the other moments act-ing on the swashplate about the Vaxis 348 when the main pump is pumping .at maximum pressure, so that any increase in pressure above `the maximum will rotate the cam member in` a stroke-decreasing direction in substantially the same manner .as .the embodiment of the invention shown and described with respect to FIG. 5. The moments acting :on the svwashplate or cam member 346 are substantially the same as those shown in FIG. 4 for the FIG. 5 embodiment of the present invention. The pistons 340 aire of lighter construction than conventionally employed in order to .reduce the inertia moment at the maximum displacement position of the cam member 346 so that the maximum inertia moment approximately equals the change in uid pressure moment from zero displacement to maximum displacement, each being shown as appnoximately iive inch-pounds in FIG. 4. ln this manner, the strokeincreasing inertia moment cancels the increase in fluid pressure momientiirom Zero to maximum displacement land the substantially constant spring moment balances the resulting lfluid pressure moment. Therefore, if the pressure in the high pressure passage, ie., one of passages 337 or 338, increases above the maximum fluid pressure for 'which the spring 44.6 is designed, the cam member 346 will rotate in a stroke-reducing direction and thereby reduoe the flow delivered by the pump and reduce the pressure in the high pressure passage to the desired maximum.

:It should be understood that in the embodiment of FIGS. 6, 7 and 8, the transmission may be employed for driving a vehicle such as, for example, a garden tractor or the like, having one or more propelling Wheels appropriately associated with motor 311. In such an installat-ion, pressure limiting control 320 will function not only during relatively constant speed vehicle openation, but also importantly during acceleration and deceleration of the vehicle as a limit control preventing excessive acceleration land deceleration which might tend to throw an operator from the vehicle.

During .accelenaation of the tnansmission when the pump 310 is pumping, the pressure limit-ing con-trol functions as described .above During deceleration of the transmission, when the pump is motoring, by virtue of the overrunning tendency of the motor, pumping, the fluid pressure moment on the pump cam is stroke increasing rather than stroke decreasing, as explained in Serial No. 113,697, now Patent No. 3,230,893, and therefore opposing deceleration, along with the stroke increasing piston inertia moment, so that when the two moments exceed the force of the spring, movement of the pump cam toward neutral is retarded.

I claim:

l. A multiple piston hydraulic pump, comprising; a port member having inlet and outlet passages-therein for receiving and delivering Huid, a cylinder block rotatable relative to said port member and having a plurality of cylinders therein successively communicating with said passages during said relative rotation, pistons slidably mounted in said cylinder block, -a cam member pivotally mounted adjacent said cylinder block on an axis and connected to said pistons for reciprocation thereof, the cam pivot axis being positioned so that the pistons cause a stroke reducing fluid pressure moment on the cam member, said cam member being movale from a neutral position where the pistons will not reciprocate to a maximum displacement position on one side of neutral, manually operable means for positioning the cam member in any desired position between neutral position and said maximum displacement position, and resilient means `for continuously urging said cam member in a stroke increasing direction against the stroke reducing piston forces, said resilient means being operable and constructed to just balance the stroke reducing piston forces produced at a predetermined maximum desired pressure in all positions of the cam member whereby the cam member will move toward said neutral position if said forces increase above said predetermined value.

2. A i'luid pressure hydraulic unit, comprising: a valve member having inlet and outlet passages therein, a block having a plurality of cylinders therein and being rotatable relative to said valve member whereby the cylinders successively communicate with said passages during said rotation, pistons slidably mounted in said cylinders, a cam member pivotally mounted adjacent said cylinder block about an axis intersecting the axis of the block and connected to said pistons for reciprocation of said pistons during said rotation, said pivot axis being positioned so that the pistons exert a fluid pressure moment on said cam member tending to rotate said cam member in a stroke-decreasing direction, said pistons exerting a moment due to the piston inertia force on said cam member tending to rotate said cam member in a stroke-increasing direction, said cam member being movable from a neutral position to a maximum displacement position on one side of neutral, manually operable means for positioning the cam member in any desired position between neutral position and said maximum displacement position, a resilient means urging said cam member in a stroke-increasing direction in all positions of said cam member, said resilient means being of suicient strength to produce a moment on the cam member at the no-stroke position of said cam member equal to the moment the maximum desired fluid pressure exerts on said cam member when the cam member is near neutral position, said resilient means producing a moment on the cam member which decreases with increasing displacement to compensate for the piston inertia moment which increases with increasing displacement whereby the cam member will move in a stroke-decreasing direction when the fluid pressure moment exceeds the maximum desired in all positions of the cam member.

3. A multiple piston pump, comprising: a valve member having inlet and outlet passages therein, a rotatable cylinder block engaging said valve member and having a plurality of axial cylinders therein, pistons slidably mounted in said cylinders for receiving and expelling fluid through said passages, a pivotally mounted cam member adjacent said cylinder block on an axis intersecting the cylinder block axis and connected to said pistons to reciprocate said pistons, said cam member being movable from a zero stroke position to a maximum stroke position, the cam pivot axis being positioned so that the pistons cause a stroke reducing luid pressure moment on the cam member; and a pressure-limiting device for preventing excessive pumping pressure includ- -ing an adjustable threaded stop engaging a portion of the p cam member and preventing movement of the cam member toward the maximum stroke position, said stop being movable to position said cam member in any desired 1position between zero stroke and maximum stroke, and a coil spring connected to said .cam member and urging said cam member in a stroke increasing direction `against said stop in all adjusted positions thereof, said pistons exerting a fluid pressure moment against said cam member urging said cam toward zero stroke and exerting an inertia moment on said cam member urging said cam toward said maximum stroke position, said coil spring having sufcient force to exert a moment on the cam member equal to said liuid pressure moment at the maximum desired Huid pressure near the zero stroke position of the cam member, said cam spring having sufficient force to exert a moment on the cam member equal to the algebraic sum of the inertia moment and the tluid pressure moment at maximum fluid pressure at said maximum stroke position, said spring having a linear force to length characteristic so that the moment on the cam member produced thereby decreases linearly with increasing pump displacement, whereby said cam will move away from said` stop when the maximum desired fluid pressure is exceeded regardless of the initial position of the cam member.

4. A hydraulic pump, comprising: a valve member having inlet and outlet ports therein, a cylinder block rotatable relative to said valve member and having cylinders therein serially communicable with said ports, pistons slidable in said cylinder block for receiving `and expelling iluid through said ports, a cam member connected to said pistons to reciprocate said pistons in the cylinders, said cam member being movable from a zero stroke position to maximum stroke positions on either side of said zero stroke position, and pressure limiting means preventing excessive pumping pressure including a threaded abutment for adjusting the position of said cam member to any desired position on either side of said zero position, a spring mounted on said cam member having two arms each engaging one side of said abutment.

5. A pump as defined in claim 4 wherein said pistons exert a iiuid pressure moment against said cam member urging said cam toward said Zero stroke position Iand exert an inertia moment on said cam member urging said cam member toward one of said maximum stroke positions, said pistons being constructed so that the change in the inertia moment from the zero stroke position to the maximum stroke position is equal and opposite to the change in inherent fiuid pressure moment from the nostroke to maximum stroke positions on both sides of neutral, said spring arms having sufiicient force to exert a moment on the cam member equal to said inherent fiuid pressure moment at maximum desired fiuid pressure at the zero stroke position of the cam member, said spring arms being constructed to exert a substantially constant moment on the cam member in all positions thereof so that the spring moment is equal to the algebraic sum of the inertia moment and the fluid pressure moment produced by the maximum uid pressure in all cam member positions whereby said cam member will move away from said abutment when the maximum desired Huid pressure is exceeded regardless of the initial position of the cam member.

6. A reversible hydraulic transmission comprising: a multiple piston pump having inlet and outlet ports, a motor having inlet and outlet ports, conduits connecting said ports each adapted to carry high pressure iiuid, a pivotally mounted cam member in said pump adapted to reciprocate the pump pistons, said cam member having a zero stroke position and maximum stroke positions on either side of said zero stroke position whereby either of the conduits may deliver high pressure fluid, a pressure limiting device for limiting the maximum pressure in said conduits including a torsion coil spring mounted on the pivotal axis of said cam member and having spring arms extending therefrom and :crossing one another, a spacer between said arms for maintaining the arms apart, an adjustable threaded member having an annular abutment engaging each of said arms whereby the cam member may be adjusted and excessive conduit pressure will cause the cam member to move toward said zero stroke position.

7. A multiple piston hydraulic unit, comprising: a port member having inlet and outlet ports therein for receiving and delivering fluid, a cylinder block rotatable rela-- .tive to said port member and having a plurality of axial cylinders therein adapted to serially communicate with said ports, pistons slidably mounted in said cylinder block, a cam member pivotally mounted about an axis which intersects the axis of rotation of the cylinder block for varying the stroke of the hydraulic unit, said cam member being movable from a neutral position to maximum displacement positions on both sides of neutral, manually operable means for positioning the cam member in any `desired position between said maximum displacement positions, said pistons exerting a iluid pressure moment and a piston inertia moment on the cam member which vary with changes in displacement; and pressure limiting means for preventing fluid pressure in excess of a predetermined maximum including first resilient means for urging the cam member in one direction in opposition and equal to the net moment caused by the maximum fluid pressure moment and the inertia moment when the cam member is in any position lon one side of neutral, and second resilient means for urging the cam member in the other direction in opposition and equal to the net moment caused the maximum fluid pressure moment and the inertia moment when the cam member is in all positions on the other side of neutral, whereby the cam member will overcome the resilient means and reduce system pressure when the maximum pressure is exceeded.

8. A multiple piston hydraulic unit as defined in claim 7 wherein said hydraulic unit is adapted to operate as a pump; said rst and second resilient means acting in a stroke increasing direction and including a trunnion fixed to said cam member and extending therefrom, a projection extending from and fixed with respect to said trunnion, a manual control handle mounted for free rotation about the axis of rotation of the cam member, a torsion spring surrounding the trunnion and having parallel ends engaging the sides of both the projection and the control handle for normally maintaining said projection in alignment with the control handle, said ends of the spring permitting the projection and the cam member to pivot away from said control handle in a stroke decreasing direction when an excessive uid pressure is produced in either of the ports.

9. A multiple piston hydraulic pump as defined in claim 8 and further including normally aligned grooves in the sides of the projection and the :control handle adapted to receive the parallel ends of the torsion spring.

10. In a hydrostatic transmission, a hydraulic motor adapted to be connected for driving a vehicle, a hydraulic pump, conduit means connecting the pump and motor for delivering fiuid from the pump outlet to the motor inlet and returning fluid from the motor outlet to the pumpl inlet, said pump comprising, a valve plate having inlet and outlet ports therein respectively communicating with the pump inlet and pump outlet, a cylinder block rotatable relative to the port plate and having a plurality of axial cylinders `therein adapted to serially :communicate with said ports, pistons slidably mounted in said cylinders, a cam member pivotally mounted about an axis intersecting the cylinder block axis for varying the stroke of the pistons, said cam member being movable from a neutral position to a maximum displacement position, said pistons exerting a stroke increasing piston inertia moment on the cam member and exerting a stroke reducing iiuid pressure moment on the cam member during pumping and a stroke increasing fluid pressure moment on the cam during motoring, a manually oper-able control handle for varying the angle of the cam member in any desired position, and an acceleration and deceleration control including double-acting spring means resiliently yieldably connecting the control handle and the cam member to normally effect movement of the cam member proportionately with movement of the control handle, said spring means being yieldable during acceleration when the net fiuid pressure moment exceeds the force of the spring means thereby to retard acceleration, and being yieldable during deceleration when the fluid pressure moment and the piston inertia moment exceed the force of the spring means thereby to retard deceleration.

11. A combination as defined in claim 10 wherein said cam member is movable from neutral position to maximum displacement positions on opposite sides of neutral.

12. In a hydrostatic transmission, a hydraulic motor adapted to be connected for driving a vehicle, a hydraulic pump, conduit means connecting the pump and motor for delivering fluid from the pump outlet to the motor inlet and returning uid from the motor outlet to the pump inlet, said pump comprising, a valve plate' having inlet and outlet ports therein respectively communicating with the pump inlet and pump outlet, a cylinder block rotatable relative to the port plate and having a plurality of axial cylinders therein adapted to serially communicate with said ports, pistons slidably mounted in said cylinders, a cam member pivotally mounted about an axis intersecting the cylinder block axis for varying the stroke of the pistons, the cam pivot axis being positioned so that the pistons cause a stroke reducing fluid pressure moment on the lcam member, said cam member being movable from a neutral position to a maximum displacement position, said pistons exerting a stroke increasing piston inertia 3,282,225 i3 14 moment `on the cam member and exerting a stroke reduc- References Cited by the Examiner ing Huid pressure moment on the cam member, a manu- UNITED STATES PATENTS tally operable :control handle for varying the angle of the mally constraining the cam member to move with the FOREIGN PATENTS control handle in a stroke increasing direction, said spring 645,947 7/1962 Canada,

means being constructed to balance the net stroke reducing piston forces in all positions of the cam member and MARK NEWMAN, Pl'l'mm'y Examinerbeing yieldable yduring lacceleration when the net fluid 10 SAMUEL LEVINE Examl-en pressure moment exceeds the force of the spring means thereby to retard acceleration. R. M. VARGO, Asszstant Examzner. 

1. A MULTIPLE PISTON HYDRAULIC PUMP, COMPRISING; A PORT MEMBER HAVING INLET AND OUTLET PASSAGES THEREIN FOR RECEIVING AND DELIVERING FLUID, A CYLINDER BLOCK ROTATABLE RELATIVE TO SAID PORT MEMBER AND HAVING A PLURALITY OF CYLINDERS THEREIN SUCCESSIVELY COMMUNICATING WITH SAID PASSAGES DURING SAID RELATIVE ROTATION, PISTONS SLIDABLY MOUNTED IN SAID CYLINDER BLOCK, A CAM MEMBER PIVOTALLY MOUNTED ADJACENT SAID CYLINDER BLOCK ON AN AXIS AND CONNECTED TO SAID PISTONS FOR RECIPROCATION THEREOF, THE CAM PIVOT AXIS BEING POSITIONED SO THAT THE PISTONS CAUSE A STROKE REDUCING FLUID PRESSURE MOMENT ON THE CAM MEMBER, SAID CAM MEMBER BEING MOVABLE FROM A NEUTRAL POSITION WHERE THE PISTONS WILL NOT RECIPROCATE TO A MAXIMUM DISPLACEMENT POSITION ON ONE SIDE OF NEUTRAL, MANUALLY OPERABLE MEANS FOR POSITIONING THE CAM MEMBER IN ANY DESIRED POSITION BETWEEN NEUTRAL POSITION AND SAID MAXIMUM DISPLACEMENT POSITION, AND RESILIENT MEANS FOR CONTINUOUSLY URGING SAID CAM MEMBER IN A STROKE INCREASING DIRECTION AGAINST THE STROKE REDUCING PISTON FORCES, SAID RESILIENT MEANS BEING OPERABLE AND CONSTRUCTED TO JUST BALANCE THE STROKE REDUCING PISTON FORCES PRODUCED AT A PREDETERMINED MAXIMUM DESIRED PRESSURE IN ALL POSITIONS OF THE CAM MEMBER WHEREBY THE CAM MEMBER WILL MOVE TOWARD SAID NEUTRAL POSITION IF SAID FORCES INCREASE ABOVE SAID PREDETERMINED VALUE. 